The present invention relates to quantitative analysis of image data. More specifically, the invention relates to quantification of extensional features of structures of an imaged subject from image data representing a two-dimensional or three-dimensional image. In a particular embodiment, the invention may be used for analysis of tomographic image data for quantification of stenosis in vascular structures, such as arteries.
Various imaging modalities exist for generating images that accurately represent the interiors of objects. Tomography (or computed tomography, abbreviated “CT”) is a class of such modalities. Unlike direct images (such as photographic images, conventional radiographs, etc.), a tomographic image is generated by reconstructing image data from projection data. The “projection data” are data that represent properties of, for example, illuminating rays transmitted through the object along numerous non-parallel paths. Reconstruction of the projection data into image data may be carried out through an appropriate computational process for image reconstruction.
In a typical case, a relevant set of projection data will represent attenuation of illuminating rays that travel along different paths in a specific plane. Generally the plane is transverse to a specified axis through the object and intersects the axis at a corresponding axial position. In the simplest case the plane is perpendicular to the specified axis, and the reconstructed image is a two-dimensional image representing the intersection of the plane with the structural features of the object.
Such an image is “tomographic” because it is obtained from projections through the object in many different directions generally parallel to the plane of the image. Although the image must be reconstructed from the projection data, the orientation of the directions of projection ensures that the reconstructed image substantially depicts only those features of the object coincident with the specified plane. In contrast, a conventional radiographic image (whether generated by analog or digital detection) is obtained directly from a projection generally perpendicular to the image plane. The conventional radiograph therefore depicts a superposition of all the structures of the object between the source and the detection surface (e.g., the film).
More generally, as used herein, the term “tomographic image” will mean any image generated by a process of tomography. A three-dimensional image of the interior of the object is therefore a “tomographic image,” also, if reconstructed from projection data. A tomographic image depicting a cross section of the object is sometimes called a tomographic “slice” image.
A slice image may be reconstructed from projection data corresponding to a particular transverse plane through the object and may represent the features of the object lying in that plane. Alternatively, using multi-planar reconstruction (“MPR”), the projection data for several different planes may be combined to generate a slice image corresponding to yet another plane transverse to all of the projection data planes. A feature common to both types of slice image, as well as to three-dimensional images, is that each data element (pixel or voxel) represents a material property (such as attenuation of the illuminating rays) at a specific point of the object.
The projection data are generated in a process that begins by illuminating the object from numerous different directions and collecting detector data representing material characteristics of the object. It is well known that tomographic imaging can be performed by illumination with any of various types of reflected or transmitted energetic rays or waves. The source of the illuminating rays or waves is often outside the object, but can also be within the object (in so-called emission tomography). Usually the image reconstruction process comprises a computationally intensive sequence of calculations to convert the projection data into image data representing an accurate and recognizable image of the object's interior.
Medical applications of tomography have become commonplace and are widely known to provide images of almost photographic detail. A trained physician can draw detailed diagnostic conclusions from a single tomographic slice image. More generally, tomographic imaging has become a useful tool in such diverse areas of application as nondestructive testing, microseismic mapping of underground geological formations, and three-dimensional imaging with electron microscopy.
In the context of medical applications, the challenges of early diagnosis and treatment of disease continue to demand improved assessment methods. For example, serious limitations have continued to exist in the available approaches for assessment of vascular disease. An example is the assessment of vascular stenosis. Vascular disease, principally in the form of atherosclerosis leading to vessel stenosis, is a leading cause of tissue infarction and ischemia.
Here, “stenosis” means an abnormal narrowing of the vessel lumen, which is the cavity that is enclosed by the vessel walls and through which the circulating blood flows. Stenosis reduces the vessel's capacity to convey blood and thus can seriously reduce perfusion in the affected organ. The most common cause of vascular stenosis is the accumulation of atherosclerotic plaques on the interior wall of the vessel.
Several imaging modalities have been available for assessing vascular disease. The standard technique for assessment of atherosclerotic stenosis is angiographic catheterization. This technique typically entails the placement of a catheter in a branch of the affected vessel, injecting a radio-opaque contrast agent while X-ray fluoroscopic images are acquired using an image intensifier or a digital detector array.
Catheterization procedures are effective but are also highly invasive. Moreover, conventional techniques for assessment by catheterization involve acquiring two dimensional (2D) projection images of the 3D vasculature. Hence the data that is acquired may be confounded by structures in the chest or by contortions of the vessels themselves.
Ultrasound imaging has been suggested for assessment of vascular stenosis. Intravascular ultrasound (IVUS) has been implemented with some success to acquire dynamic images of the affected vasculature. However, as with conventional angiographic catheterization, IVUS techniques are highly invasive. Noninvasive ultrasound techniques produce images that are affected by such variables as the probe placement between other body structures and the proximity of the probe to the vasculature of interest. These factors, and the need for a skilled operator to collect reliable data, raise important issues for reliable and repeatable assessment of vascular disease.
Both x-ray tomography (XCT) and magnetic resonance (MR) tomography have been proposed for assessment of vascular stenosis. For example, U.S. Pat. No. 5,757,877, issued to Wilting, discloses a technique for estimating stenosis in renal arteries from tomographic slice images. The technique applied therein obtains a quantitative assessment of stenosis by estimating the diameter of the affected vessel. Using MPR, a slice image is reconstructed to show a longitudinal cross section of the vessel in the region of interest. An operator selects the orientation of the slice image to be reconstructed, whereby a portion of the longitudinal axis of the vessel is included in the image plane. The diameter of the vessel at a selected position in the selected image plane can then be measured by known methods.
It has been found that assessment of stenosis by measurements of vessel diameter is sensitive to a variety of operator-dependent factors. For example, an estimate of vessel diameter will depend critically on the selection of the image plane in which the diameter is measured. A misalignment of the tomographic slice (i.e., away from the vessel's longitudinal axis) may significantly affect the measured value of the diameter. The operator's skill and experience in selecting the best image plane for the multi-planar reconstruction therefore directly affects the accuracy of the assessment result.
The accuracy of the assessment also depends critically on the selection of the longitudinal position at which the vessel diameter is measured. Even an accurate diameter measurement will provide a reliable estimate of stenosis only when the measurement is taken at the position of greatest lumen narrowing. Hence, the selection of the longitudinal position at which to measure the vessel diameter also strongly affects the measured diameter value.
If the plane of the longitudinal cross section is selected carefully, then a trained technician or physician can visually identify the approximate longitudinal position of greatest narrowing. An image transverse to the vessel's longitudinal axis would be unsuitable for this purpose, because diameters along the longitudinal axis could not be compared. Consequently, it has been generally understood in the art that stenosis should be assessed from images oriented along the vessel's longitudinal axis. Indeed, U.S. Pat. No. 4,987,585, to Kidd et al., notes that stenosis may not even be apparent from images oriented transversely to the vessel's longitudinal axis.
Diameter measurements are also affected by the degree to which the vessel lumen diverges from roundness. If the lumen is oblong or irregular in cross section, then the lumen diameter will vary with the angular orientation of the selected image plane relative to the longitudinal axis of the vessel. The aforementioned patent to Kidd, et al., illustrates that this dependence of measured diameter on angular orientation of the plane of measurement is well known in the art. The conventional solution to this problem has been to generate several longitudinal cross sectional images of the constricted area at different angles.
The foregoing considerations also illustrate that quantitative assessment of structural features represented in tomographic images has been essentially a subjective process. Medical tomographic images and conventional radiographic images (whether generated by direct film exposure of by digital imaging methods) have been treated in much the same manner. In the typical procedure, an expert trained in medical image interpretation (a “reader”) visually examines the image.
The aforementioned patent to Wilting shows that the reader may use computational aids to obtain measurements of vessel diameters. However, the foregoing observations illustrate some of the sources of assessment error that are associated with estimating the degree of stenosis based on lumen diameters. For example, if the lumen cross section is not round in the constricted area (a common situation), then a useful estimate of lumen size will still entail combining several diameter measurements from different images. The resulting quantitative assessment will depend on correctly selecting the particular images in which to make the measurements and the measurement locations in the selected images.
There has been recent interest in using volumetric data generated by other modalities to quantify vascular stenosis, such as magnetic resonance imaging (MRI). MRI uses magnetic polarization of imaged tissue and magnetic field excitation of the polarized atoms (e.g., by radio waves) to acquire volumetric data. As in CT, the volumetric data are then reconstructed to generate a three-dimensional (3D) model of the structure or organ of interest.
Using MR images, an organ's vasculature may be segmented from the surrounding tissues and the amount of stenosis determined, typically through visual examination of the generated images by a trained reader. MRI scans to produce medical images may entail many minutes to acquire volumetric data corresponding to the region of interest
More generally, the greatest advantage of MR imaging for assessment of vascular stenosis has been the increased resolution and accuracy of the resulting images, relative to conventional angiographic techniques. These benefits have tended to increase the accuracy and repeatability of subjective assessment. Hence, the MR imaging modality has been considered as an approach by which the results of convention assessment methods could be improved.
CT imaging systems, MRI systems, and other imaging systems are or may be capable of rendering volumetric images from which segmentation of the vessel from adjoining tissue may be performed. However, frequently the reconstructed images will depict the structure of interest in an inconvenient orientation for purposes of examining particular structural features. The general result has been reduced precision for the assessment to be performed.
It follows that a tangible need continues to exist for a technique by which extensional features in images, such as diagnostic images, can be reliably and objectively quantified. Desirably, such quantification techniques could be readily implemented with the existing technology for tomographic imaging, as well as with other imaging modalities. Even more desirably, the quantitative assessment results thereby obtained would be relatively insensitive to confounding factors such as image noise, image artifacts, and operator selection of imaging parameters.
Ideally, such an approach could be applied to provide improved assessments of vascular stenosis, such as arterial stenosis, but could also be applied in a broad range of medical and non-medical application areas. The desired techniques would be readily applied in a range of areas where quantification of structural dimensions or other extensional features is needed and is currently unfeasible or subject to poorly constrained sources of error.
Here, the term “extensional feature” means a dimension, extent, characteristic, or property of an imaged subject that can be quantified by measuring a structural feature of the subject. An extensional feature typically will be a spatial extent of the structural feature, such as length, width, height, area, or volume. Alternatively, an extensional feature may be a non-spatial feature related to such a spatial extent of the structure by a known or determinable relationship.
A particular need exists for imaging and analysis methods and systems, such as x-ray CT systems with or without ancillary image processing systems, by which stenosis in vasculature structures can be reliably assessed. Such an approach would most desirably make evaluation of vascular stenosis a routine procedure. The substantial clinical benefits of such a technology would include, for example, significant reduction in variability of results due to differences of judgment between different readers. Further, because any necessary contrast medium could be injected into the peripheral vasculature, the desired technique would be only minimally invasive.